How can I prevent the body object slipping through the clothing object?
For this to work nicely you have to either:


have the clothes as one mesh with the skin for good results with automatic weighting
create the weights in Maya with Geodesic Voxel Binding
weight just the skin, then copy (project) the weights with Data Transfer modifier to the clothes. This way they won't slide (hopefully)


create a shell around the clothes and skin, rig the shell with armature, drive both the skin and clothes inside the shell through Mesh Deform modifier